Add USB CDC and loop-back test

stm32f103c8t6/src/usb_vendor.c

@@ -0,0 +1,20 @@
+#include "usb_vendor.h"
+
+void USBVendor_HandleDeviceSetup(USB_SetupPacket_t *sp,
+    const void **reply_data, int *reply_length, uint8_t *reply_response)
+{
+    USBVendor_Command_t command = sp->bRequest;
+
+    *reply_data = NULL;
+    *reply_length = 0;
+
+    switch(command)
+    {
+        case USB_VENDOR_COMMAND_NOP:
+            *reply_response = USB_EP_TX_VALID;
+            break;
+
+        default:
+            *reply_response = USB_EP_TX_STALL;
+    }
+}